Glass manufacturing apparatus are known to produce glass sheets, for example, by a fusion down draw process. U.S. Pat. No. 8,627,684 to Shultz et al. that issued on Jan. 14, 2014 discloses an example glass manufacturing apparatus with a lower pull roll apparatus having a master motor to rotate a lower pair of rolls at a constant angular velocity. The glass manufacturing apparatus further includes an upper pull roll apparatus with upper slave motors configured to rotate an upper pair of rolls at torques that match a predetermined percentage of the measured torque of the master motor of the lower pair of rolls.
The master and slave configuration of the lower and upper pull roll apparatus of the Shultz et al. patent can be beneficial under various process applications. However, perturbations from the glass ribbon growth and sheet formation may propagate to the upper pair of rolls. For example, FIG. 1A illustrates an example graph of a master and slave configuration where the vertical “Y-axis” is force (pounds) and the horizontal “X-axis” is time (minutes:seconds). One plot 101 represents the force being applied to the glass ribbon by the lower rolls while the other plot 103 represents the force being applied to the glass ribbon by the upper rolls. As shown, each plot 101, 103 includes a saw-tooth force pattern with a first force pattern 105 representing glass ribbon growth and a second force pattern 107 representing separating of a glass sheet from the glass ribbon.
U.S. Patent Application Publication No. 2013/0133371 that published on May 30, 2013, to Burdette et al. (hereinafter the Burdette et al. publication), discloses an example glass manufacturing apparatus including a forming device configured to produce a glass ribbon, a pull roll device, and a control device. The control device of Burdette et al. is configured to independently operate an upper pull roll apparatus and a lower pull roll apparatus such that the upper pull roll apparatus rotates with a substantially constant torque and the lower pull roll apparatus rotates with a substantially constant angular velocity. The independent operating configuration of the upper and lower pull roll apparatus of the Burdette et al. publication can also be beneficial under various process applications.
For example, FIG. 1B illustrates a graph of an example independent operating configuration of a pull roll device, representative of the Burdette et al. publication, where the vertical “Y-axis” is force (pounds) and the horizontal “X-axis” is time (minutes:seconds). One plot 111 represents the force being applied to the glass ribbon by the lower pull roll apparatus while the other plot 113 represents the force being applied to the glass ribbon by the upper pull roll apparatus. As shown, the plot 113 remains substantially constant as the upper pull roll apparatus rotates with a substantially constant torque and thus applies a substantially constant force to the glass ribbon while the plot 111 varies as the lower pull roll apparatus rotates with a substantially constant angular velocity and thus applies a varying force to the glass ribbon. As further shown, in contrast to the plot 103 of the Shultz et al. patent, the plot 113 is independent from the plot 111 as the force being applied to the glass ribbon by the upper pull roll apparatus is independent of the force being applied to the glass ribbon by the lower pull roll apparatus.
As further shown in FIG. 1B, a third force pattern 115 of the plot 111 represents, for example, the changing force as the glass ribbon increases in length while the fourth force pattern 117 represents, for example, the sudden change in force that occurs during separation of a glass sheet from the glass ribbon. During the same period of time, the constant torque of the upper pull roll apparatus can maintain a substantially constant force to the glass ribbon. As such, force disturbances at or below the lower pull roll apparatus can be prevented from being transmitted up the glass ribbon into the setting zone where stress concentrations and corresponding surface defects may be undesirably frozen into the glass ribbon.
However, a change in a characteristic of the glass ribbon, for example, can produce a corresponding change in an operating condition of at least one of the upper pull roll apparatus which rotates with a substantially constant torque and the lower pull roll apparatus which rotates with a substantially constant angular velocity. The corresponding change in the operating condition can affect, for example, a quality of the glass ribbon.
For example, FIG. 1C, illustrates a graph of an example independent operating configuration of a pull roll device, representative of the Burdette et al. publication, where the vertical “left Y-axis” is a change in force (pounds), the vertical “right Y-axis” is a change in viscosity of the glass ribbon at the root (%), and the X-axis is time (minutes:seconds). One plot 121 represents the change in force being applied to the glass ribbon by the lower pull roll apparatus while the other plot 123 represents the change in force being applied to the glass ribbon by the upper pull roll apparatus. As shown, the plot 123 remains substantially constant as the upper pull roll apparatus rotates with a substantially constant torque and thus applies a substantially constant force to the glass ribbon while the plot 121 varies as the lower pull roll apparatus rotates with a substantially constant angular velocity and thus applies a varying force to the glass ribbon for reasons discussed with respect to plot 125 below.
FIG. 1C also represents how a change in a characteristic of the glass ribbon over a period of time can change the force differential applied to the glass ribbon by the upper pull roll apparatus and the lower pull roll apparatus. In particular, FIG. 1C demonstrates how a change in “root viscosity” (i.e., the viscosity of the glass ribbon at the root of the forming wedge) over a period of time can change the force differential applied to the glass ribbon by the upper pull roll apparatus and the lower pull roll apparatus. For instance, FIG. 1C demonstrates that as the change in root viscosity (plot 125) increases over a period of time, the corresponding force being applied to the glass ribbon by the lower pull roll apparatus (plot 121) likewise changes over the same period of time. However, as demonstrated by plot 123, the force being applied to the glass ribbon by the upper pull roll apparatus remains constant over the period of time since the upper pull roll apparatus rotates with a substantially constant torque. Consequently, a first force differential 127 at a first time (i.e., 12:00) is significantly less than a second force differential 129 at a subsequent second time (i.e., 12:57) due to a corresponding increase in root viscosity. Thus, independently operating the upper pull roll apparatus and the lower pull roll apparatus such that the upper pull roll apparatus rotates with a substantially constant torque and the lower pull roll apparatus rotates with a substantially constant angular velocity (e.g., as set forth by the Burdette et al. publication) may result in a significant change in the force differential over time in response to a change in a characteristic of the glass ribbon (e.g., root viscosity) over the same period of time.